1. Field of the Invention
The present invention relates to an optical transmission device using return-to-zero optical pulses as transmission optical pulses, which Is capable of enhancing a transmission distance as well as a wavelength range with satisfactory transmission characteristics and enabling an easy realization of the wavelength division multiplexing optical transmission. The present invention also relates to a wavelength division multiplexing (WDM) optical transmission apparatus and an optical transmission system formed by utilizing such an optical transmission device.
2. Description of the Background Art
In recent years, due to the advance of the optical amplification technique, the optical fiber communication technique has been progressed to realize a very long distance communication, and it is now becoming possible to realize the transoceanic communication using optical submarine cables without requiring regenerators. As a transmission scheme for enabling a high speed, large capacity realization of such an optical communication using optical fibers, an optical soliton transmission using return-to-zero optical pulses as transmission optical pulses has been attracting much attention.
However, this optical soliton transmission is associated with a random variation of optical signal wavelength due to optical amplifier noises and a nonlinear effect of optical fibers, and a random jitter (known as Gordon Haus dynamic jitter) of pulse arrival time at a receiving terminal due to dispersion by optical fibers and there has been a problem that a transmission distance is limited because of these random timing jitters.
As a solution to resolve this problem, there has been a proposition of a scheme in which a mean dispersion of the system is made smaller by periodically compensating the accumulated dispersion of transmission fibers to a nearly zero level using dispersion compensation fibers, so that a random jitter of optical signal wavelength due to optical amplifier noises and a nonlinear effect of optical fibers will not be converted into the dynamic timing jitter.
However, according to the analysis made by the present inventors, this scheme limits the optimum wavelength range so that, in a case of multiple wavelengths transmission, the accumulated residual dispersion becomes large for long wavelength signals and it is difficult to realize a long distance transmission.
In addition, at a wavelength with the optimum dispersion compensation for which a mean dispersion is small, this scheme causes an excessive spectrum spread, so that there is a need to use a large wavelength interval in order to avoid interferences among different wavelengths, even when the dispersion slope of optical fibers is compensated to realize a nearly optimum value for the residual dispersion at each wavelength, and for this reason, there is a problem that it is difficult to utilize the optical amplifier bandwidth efficiently.